1. Technical Field
The present invention relates to a peptide analog and to methods of using same to attenuate the effects of drug addiction, drug tolerance, drug dependence or of abstinence syndrome.
2. Background Information
The octapeptide NPFF (Phe-Leu-Phe-Gln-Pro-Gln-Arg-Phe-amide (SEQ ID NO:1)-amide) was originally isolated by Yang et al from bovine brain (Yang et al, Proc. Natl. Acad. Sci., 82:7757-7761 (1985)). It has also been referred to as "morphine-modulating peptide" or "FMRF-NH.sub.2 -like mammalian octapeptide" (Panula et al, Med. Biology, 65:127-35 (1987)) or Neuropeptide FF (Kivipelto et al, Journal of Comparative Neurology). There are reasons to suspect that NPFF may be an "anti-opiate peptide": NPFF is localized in several brain regions rich in endogenous opioids (Ferrarese et al, Regulatory Peptides, 13:245-52 (1986); Panula et al, Med. Biology, 65:127-35 (1987)), is released from the brain by morphine infusion (Tang et al, Proc. Natl. Acad. Sci., 81:5002-5 (1984)), and potently antagonizes analgesic effects of morphine and certain endogenous opioid peptides (Tang et al, Proc. Natl. Acad. Sci., 81:5002-5 (1984); Yang et al, Proc. Natl. Acad. Sci., 82:7757-7761 (1985); Yang et al, Prog. Clin. Biol. Res., 192:313-22 (1985)). IgG from NPFF antiserum augments morphine and stress-induced analgesia (Kavaliers et al, Peptides, 10:741-5 (1989)).
There is also evidence that NPFF may participate in opiate tolerance and dependence. IgG prepared from FMRFa antiserum cross-reacts with NPFF and interferes with morphine tolerance (Tang et al, Proc. Natl. Acad. Sci., 81:5002-5 (1984)). NPFF levels in CSF are markedly increased in opiate dependent rats as compared with non-dependent rats (Malin et al, Peptides, 11:969-972 (1990)). NPFF (2 .mu.g i.c.v.) precipitates opiate abstinence syndrome in morphine-dependent rats (Malin et al, Peptides, 11:277-280 (1990)), and NPFF (15 .mu.g i.c.v.) induces a quasi-morphine-abstinence syndrome (QMAS) in opiate-naive rats (Malin et al, Peptides, 11:277-280 (1990)) (see also Guzman et al, Neuropeptides 14:253-261 (1989); Majane et al, Peptides, 8:657-662 (1987); Majane et al, Peptides, 9:1137-1144 (1988)). Third ventricle infusion of IgG from NPFF antiserum reverses opiate dependence, as evidenced by prevention of naloxone-precipitated abstinence syndrome in morphine-dependent rats (Malin et al, Peptides, 11:969-972 (1990)). The mechanism of action of NPFF is not understood as yet, but a recent receptor binding study in spinal cord membranes suggested that the neuropeptide binds to specific NPFF receptors. The .sup.125 I-Y8Fa binding site showed high affinity for NPFF, whereas opioid ligands failed to compete for binding (Allard et al, Brain Research, 500:169-176 (1989)).
Applicants recognized that an NPFF antagonist peptide would be useful as a probe for determining the physiological role of endogenous NPFF, as well as further ascertaining its role in opiate dependence, tolerance and abstinence. Accordingly, a NPFF analog was synthesized. A preferred embodiment of this analog differs in two respects from the NPFF sequence. First, in order to reduce receptor activation, the C-terminal Arg-Phe-amide is replaced by Arg-amide. Secondly, in order to increase resistance to aminopeptidase, the N-terminal is blocked with desaminotyrosine (daY). With both N-and C-terminals blocked, this peptide has increased enzyme resistance and receptor availability. DaY increases peptide binding affinity at molluscan FMRFa receptors and it was for this reason that it was added (see Payza, Peptides, 8:1065-1074 (1987)).
The analog of the invention is useful in acting as an antagonist in blocking the effects of NPFF, and in acting to block dependence on drugs of abuse or addiction and their subsequent abstinence syndromes. The present invention provides a method by which the NPFF analog can be employed to either enhance the efficacy of morphine treatment or to prevent drug dependence and to ameliorate the effects of abstinence syndromes.